mptcp: move sk assignment statement ahead

mptcp: move sk assignment statement ahead

If we move the sk assignment statement ahead in mptcp_nl_cmd_sf_create()
or mptcp_nl_cmd_sf_destroy(), right after the msk null-check statements,
sk can be used after the create_err or destroy_err labels instead of
open-coding it again.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-7-db8f25f798eb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>